Patent Application
20070205359
The field of the invention relates to microanalytics and more particularly to gas pumps.
Presently available gas pumps for microanalytics are relatively large and use mechanical actuators that are subject to wear and limited service life. The use of mechanical actuators creates undesirable flow pulsations that can only be reduced through bulky buffer volumes. The difficulty of fabricating and assembling such mechanical pumps is significant and contributes to their high price.
Ion drag pumps overcome many of the deficiencies of mechanical pumps. Ion drag pumps first ionize a gas and then use an electric field to attract the ions. As ions are pulled along by the electric field, they also drag along other gas molecules.
However, ion drag pumps require high ionization fields and generate relatively few ions. The high ionization fields consume a relatively large amount of energy for the number of ions produced.
In addition, the relatively few ions are relatively inefficient in dragging along other molecules because their low number causes them to interact with relatively few other molecules. In addition, the ions generate friction as they are pulled though other neutral gas molecules thereby further wasting energy.
While ion drag pumps are an improvement over mechanical pumps, they are still relatively inefficient. Accordingly, a need exists for improved pumping methods for microanalytics.
A method and apparatus are provided for pumping gas. The method includes the steps of providing an orifice with an entry diameter that is substantially larger than an exit diameter, imposing a non-uniform electric field between the entry and exit of the orifice and pumping permanent and field induced molecular dipoles towards the smaller orifice using the non-uniform electric field wherein the dipoles drift towards a direction of higher field strength within the non-uniform electric field.
a-b depicts the electronic pump of
a-b depicts the electronic pump of
Under illustrated embodiments of the invention, these dipoles can be made to drift towards the direction of higher field strength, which may be alternating current (AC) or direct current (DC), but preferably DC. By favoring microscale, low voltages and highly non-uniform field geometries, ionization is inhibited and dipole drift is increasingly favored. In addition, by relying upon molecular dipoles, the pump 10 operates on 106 to 108 more molecules than an ion drag pump.
In general, the pump 10 may be fabricated on a relatively thin (e.g., 1-10 μm) substrate 12. A through-aperture 18 having the shape of a frustrum may be created in the substrate 12 that has a taper 26 at an appropriate ratio (e.g., 3-10:1) progressing from a wide end of the aperture to a narrow end. The diameter of the narrow end 20 in one particular preferred embodiment is approximately 10 μm and the diameter of the wide end 22 is 30 μm or larger. In an even more preferred embodiment, the diameter of the narrow end 20 may be approximately 3 μm and the diameter of the wide end 22 is greater than 30 μm.
The substrate 12 may be provided with electrodes 14, 16 on opposing sides of the substrate 12 that surround opposing ends 20, 22 of the aperture 18. A power supply 24 of an appropriate voltage (e.g., 10 volts) is connected to the electrodes 14, 16 with the positive side of the power supply 24 connected to the electrode 14 on the narrow end 20 of the aperture 18. The voltage of the power supply 24 may be chosen to achieve an electric field in the range of from 104-105 V/cm.
In general, the taper 26 of the aperture 18 causes the non-uniform E-field within the aperture 18. The small size of the aperture 18 is contemplated and preferred because the E-field non-uniformity becomes commensurate with the molecular dipole dimensions of 0.2-1.2 nm, thereby reducing the voltage requirements of the power supply 24 and the formation of free electrons (corona discharge) and positive ions.
In general, the electronic pump 10 moves molecules (e.g., dipole 28) via electrostatic forces acting on permanent or E-field induced molecular dipoles. To provide an appreciation of the practicality and feasibility of the pump 10, the forces of electric fields on dipoles 28 may by compared with the forces on ions.
The net force of a non-uniform electric field, E, on the (+) and (−) electric dipole charges, q, can be characterized by the equation Fdipole=q(E2−E1), where E1 and E2 are the electric fields on opposing ends of the molecular dipole 28. In the case of the dipole 28, E1 is different than E2 because of the length, L, between the (+) and (−) charge of the dipole, in relation to the field gradient dE/ds, where S is a spatial parameter related to the non-uniform field geometry of the aperture 18. Because the dimension of L<<S=Σs, where S is the spacing between the electrodes 14, 16, the value E2 may be approximated by the expression E2=E1(1−L/S)m, depending on the location in the non-uniform field and where m is some value having the range 1≦m≦2.
In contrast to ions where qion=qe=1.6×10−19, the induced charge of a dipole 28 depends on its molecular polarizability, α. In the case of the dipole 28, the induced charge may be described by the equation, qdipoleL=αE′=A·E′/(4πNA/3), where E′ is the electric field at the molecule, which in the gas phase may be assumed equal to E, and A=molar refractivity, which is an atomic property that holds its value relatively well, regardless of the molecular bond of that atom, its gas pressure or gas phase (gas or liquid). For example, the A values for I=2.01, H=1.02, C=2.11, S=8.23, Cl=5.72, air=4.37 and C8H18=39.19 cm3/molecule.
A ratio of forces that compares dipoles with ions can be written as follows.
R=Fdipole/Fion=qdipole·E(L/S)m/qe·E=A·E·(L/S)m{L(4π·NA/3)·qe
Using values of A=4.37 for air, E=105-105 V/cm, S=10−1 μm, L=3, A=3×10−8 cm, and NA=6.02×1023 produces the result as follows,
The value of R assumes larger values in the case where for E=105 V/CM and S=1 μm. For the latter example and where m is assumed to take and even larger value (e.g., m=3), R=0.00097. In any case these results show that the forces on dipoles can be smaller but comparable to those on ions. But this is all that is needed to demonstrate the potential of this approach, especially in view of the fact that this can be done with concentrations of dipoles that are 105-107 times higher than those of ions, because the force is applied to all the molecular dipoles present in the field, rather than to an approximately 10−7 fractional concentration of ions.
Under other embodiments, the pump 10 of
In another alternate embodiment (
In the case of
In the case of
a depicts a sandwich arrangement of two substrates 12 with a dividing electrode M and outside electrodes L, S. The three electrodes S, M, L are connected to different phases.
In this case, dipoles 28 are strongly urged through the orifice 18 by a non-uniform electric field created by electrodes S, M, L. However, there is no force on the dipoles 28 between groups 102, 104, 106 because the corresponding electrodes S, M, L in each group are connected to the same phase. As a result the multistage pump of
In
Overall, the multistage and multiphase pumps of
It should be noted that the dipole attraction is insensitive to polarity, thus generating attraction towards the stronger (−) or (+) electrodes (i.e., orifices 18). Since dipoles 28 can be attracted to (+) or (−) non-uniform fields, the AC frequency of the source 300 can be adjusted to match the drift velocity and spacings between groups 102, 104, 106, 108.
a-b shows how the 3-phase voltages relate to ground or neutral (ground) and, more importantly, how they relate to each other. The top sketch (
In still further alternate embodiments, the pump 10 may be used as a valve. In this case, the voltage applied to the electrodes 14, 16 is chosen to oppose and balance an external pressure (e.g., to facilitate valve-less injection of a preconcentrated analyte from a sample gas #1 such as air into a carrier gas stream #2, such as hydrogen.
In still another embodiment, the pump 10 may be used as a selective pump. In this case, the pump 10 may be used to preconcentrate polar gas molecules (e.g., those with strong dipoles).
The pump 10 offers a number of advantages over prior devices. For example, the pump 10 saves energy over ion-drag pumps because the large unrecoverable energy required to generate ions is eliminated. The pump is more efficient than AC pumps because of the elimination of ohmic conductor and dielectric material losses.
Further, the pump 10 is non-destructive. For example, analytes are not fragmented as are molecules in ion-drag pumps, which enables the positioning of one or more pumps 10 along a separation column.
The pump 10 eliminates flow pulsations and the need for buffer volumes. Since the pump 10 relies upon an electric field for pumping, there is no mechanical noise and no mechanical wear.
A specific embodiment of an electronic pump has been described for the purpose of illustrating the manner in which one possible alternative of the invention is made and used. It should be understood that the implementation of other variations and modifications of embodiments of the invention and its various aspects will be apparent to one skilled in the art, and that the various alternative embodiments of the invention are not limited by the specific embodiments described. Therefore, it is contemplated to cover all possible alternative embodiments of the invention and any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.
